Amorphous silicon (hydrogenated amorphous silicon) is a recently developed material for making a photosensitive layer of an electrophotographic photoreceptor, and various improvements have been attained. A photoreceptor using amorphous silicon is usually prepared by forming an amorphous silicon film on a conductive substrate by glow discharge decomposition of silane gas (SiH.sub.4). The thus formed amorphous silicon film contains a hydrogen atom and exhibits satisfactory photoconductivity. The amorphous silicon photoreceptor exhibits various advantages, such as excellent wear resistance due to the high surface hardness of the photosensitive layer, high heat resistance, electrical stability, broad spectral sensitivity, and high photosensitivity, which are ideal properties needed for an electrophotographic photoreceptor.
Notwithstanding these excellent properties, an amorphous silicon photoreceptor undergoes a large dark decay due to its relatively low dark resistance. This leads to a disadvantage in that a sufficient potential cannot be obtained on charging. That is, when a charged amorphous silicon photoreceptor is imagewise exposed to light to form an electrostatic latent image and then the latent image is developed, the surface potential of the photoreceptor decays by the time of the exposure, or the charges on the non-exposed area also decay by the time to the development, resulting in a failure of keep the potential necessary for development.
The potential decay is dependent on environmental conditions and becomes conspicuous particularly under high temperature and high humidity conditions. Besides, the initial surface potential attained gradually decreases on repeated use. Such an electrophotographic photoreceptor undergoing great dark decay only produces copies having a low image density and poor reproducibility of halftone.
In order to overcome this problem, it has been proposed to form a charge blocking layer (charge injection preventing layer) comprising amorphous silicon carbide, amorphous silicon nitride, amorphous silicon oxide, etc. by plasma CVD (chemical vapor deposition) on the amorphous silicon photoconductive layer. The charge blocking layer also serves as a surface protective layer.
However, an amorphous silicon photoreceptor having such a surface layer causes image smearing or blur on repeated use, particularly under high humidity conditions, and it cannot be used in an ordinary electrophotographic process.
JP-A-2-111962 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teaches formation of a surface protective and lubricating layer mainly composed of an organic binder resin, but the proposed surface layer is too thin (10 to 300 nm) for practical use.
Further, amorphous silicon formed by plasma CVD, though having high surface hardness, is less resistant against impact and is broken more easily than a selenium photoconductive layer or an organic photoreceptor. Therefore, a photoreceptor mainly composed of amorphous silicon is likely to suffer from scratches on contact with a paper stripping click, etc., and the scratches develop white spots or black spots on reproduced images.
Furthermore, an amorphous silicon photoreceptor generally has many hemispherical defects of 1 to 30 .mu.m in diameter on its surface. On repeated use, electrical or mechanical destruction occurs at the film defects causing white spots or black spots to develop on reproduced images.